When in use, a paintbrush is in constant contact with the painter's hand. The hand comes into the most contact with the paintbrush handle, which rests on the hand between the thumb and forefinger. The paintbrush handle is held in place by the clamping action of the bony portions of the thumb and forefinger.
The result is a concentration of pressure on portions of the hand with little padding. Thus these bony portions of the hand become fatigued or bruised by the long periods of time they remain compressed against the paintbrush handle. Given that most paintbrush handles are made of wood, a stiff material, the paintbrush handle does little to spread the load.
What is needed is a handle that is soft enough to be comfortable in the hand, but matches the structural integrity of a stiffer material.